1. Field of the Invention
The present invention concerns a standard test specimen for optical emission spectrochemical analysis of metals and alloys which has a homogeneous content of metallic elements and of stable quality.
2. Description of the Prior Art
As a method capable of rapidly analyzing compositions of metals, optical emission spectrometry has been adopted for the analysis of various kinds of alloys in which the content of an element in a test sample to be analyzed is determined in accordance with a calibration curve showing the correlation between the spectral intensity ratio (absolute radiation power of an element / absolute radiation power of the base metal) and the concentration of the element in the standard sample.
In optical emission spectrometry, since the spectral intensity ratio of a sample to be analyzed is incorporated into a previously prepared calibration curve and the content of the element in the sample is determined from this curve, the precision of the analysis for the sample and its accuracy depends on the accuracy of the previously prepared calibration curve and the accuracy of the daily drift correction in the calibration curve (fluctuation of the spectral intensity ratio caused by the analyzing device). Accordingly, a standard sample of homogeneous and known element content is necessary for an accurate analysis.
By the way, as the standard sample used for such optical emission spectrometry, a disc-likeingot of 40 to 60 mm diameter.times.5 to 10 mm thickness prepared by casting a molten metal into a die is used as a standard sample after determining a standard composition value thereof by chemical analysis. Upon optical emission spectrometry, the surface of this standard sample is machined into a smooth surface and then a calibration curve is drawn based on the spectral intensity ratio caused by sparking predetermined positions in the surface. However, segregation often occurs upon solidification of the molten metal, as is usual, if a large amount of alloying elements are present. As a result, the spectral intensity varies depending on changes in the 2- or 3-dimensional positions of the surface to be sparked and, accordingly, accurate analysis is impossible with such a standard sample. In view of the above, for reducing the segregation upon solidification of the molten metal, the molten metal may sometimes be cast into an ingot rod of a small diameter by semi-continuous casting and then cutting into a disc-like shape.
However, segregation can not be overcome even if cast into an ingot rod of a small diameter and, in particular, segregation becomes more serious as the content of the element is increased. For instance, for Al-Si series alloys containing Si exceeding an eutectic point, primary Si crystals are precipitated and the segregation of such precipitated primary Si crystals makes the calibration curve and the drift correction therefor inaccurate even if it is cast into an ingot rod, making accurate analysis for the sample to be analyzed impossible.